Fuse



Nov. 9, 1948. E. H. YoNKERs t 2,453,395

FUSE

Filed Dec. 13, 1943 fZQ ai: Z50

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ELT/NG TIME /N ECNS. BY

Patented Nov. 9, 51948 UNITED STATES PATENT OFFICE FUSE Edward H. Yonkers, Chicago, Ill., assignor to Joslyn Mi'g. and Supply Co., Chicago, Ill., a corporation of Illinois Application December 13, 1943, Serial No. 514,030

(Cl. G-115) 14 Claims. 1

The present invention relates to electrical fuses and more particularly to an improved fuse link which is exceptionally well adapted for use in providing current overload protection for a power distribution transformer of low KVA capacity.

Primary fuses now commonly used for the protection of small power distribution transformers designed to operate at high primary voltages, are wholly inadequate to provide the required surge and overload protection. Thus, those available primary fuses which are of sufficiently low current rating to protect a smaller transformer from damage due to sustained secondary overload current, are easily ruptured when subjected to surge currents of insuilcient magnitude and duration to damage the transformer. More generally considered, the time-current characteristic of a conventional fuse link differs so radically from the time-overload characteristic of the usual commercial current distribution transformer that satisfactory coordination of the two characteristics is impossible. For example, a 11A KVA transformer operating at 7200 volts has a full load primary current of approximately 1A ampere. Assuming that a transformer of this size is capable of carrying 200% of its rated capacity indefinitely, a fuse link designed to blow at approximately 0.5 ampere is required in the primary circuit to insure complete protection for sustained secondary overload current. All commercial fuse links available for this type of service have current-time characteristics that are substantially flat after five or ten minutes. In other words, if they do not rupture during this interval, they will carry the current continuously under a given current condition. The transformer, on the other hand, may easily handle from 400% to 500% overload for ten minutes and may be capable of carrying 300% overload for approximately iifty minutes without damage thereto. Also, if a transformer o fwfthls type is normally subjected to a load which approximates its full load rating, it is almost certain to be subjected to momentary loads of from 300% to 500% of its full load rating when motors, or other like devices are initially energized from the secondary side of the transformer. Overloads of this character will in all cases rupture the 0.5 ampere fuse link utilized as the protective device in the primary circuit of the transformer. To avoid such fuse outages, the average load upon the transformer may be kept considerably below the full load rating of the transformer, or alternatively, a fuse link of larger current rating may be used. If the first course is follower, the transformer is not operated at its maximum efliciency, and the transformer costl in its relationship to the magnitude of the load current supplied is excessively high. If the second course is chosen, the fuse link of larger current rating will permit the transformer to be subjected to excessive current overloads of long duration with resulting damage to the transformer. In practice, the usual procedure is that of using a fuse link of sufiicient current capacity to handle the surge currents without rupture. In fact, most primary fuse links in service with transformers of one to ten KVA capacity operating at a primary voltage of 7200 volts do not afford any overload protection whatever. They function merely as disconnecting devices to sever the connection between the transformer primary Winding and the line in the event of primary winding failure.

It is an object of the present invention, therefore, to provide an improved fuse, particularly adapted for use in the high voltage primary circuit of small power distribution transformers, which will reliably protect the transformer against damage from any overload condition which may result from secondary circuit conditions and at the same time will withstand the transient disturbance which occurs on distribution lines.

It is another object of the invention to provide an improved fuse having a time-current characteristic which follows and approximates with tolerable accuracy the time-overload characteristic of the device which it is utilized to protect.

According to another object of the invention, the improved fuse link is equipped with rst and second fusible sections having different time-current characteristics, which fusible sections are so arranged and controlled as to provide overload protection over only portions of their respective characteristics, such that the first fusible section acts as a sustained current overload protective device and the second fusible section functions to provide protection against short time Overloads.

In accordance with a further object of the invention, the desired time-current characteristic is imparted to one of the fusible sections of the link by the provision of a source of heat separate from the fusible section but thermally coupled thereto by a medium having appropriate thermal conductivity.

According to still another object of the invention, an electrical heating element, arranged in series circuit relationship with the rst fusible section of the link, is utilized to deliver heat to this link section.

In accordance with a still further object of the portion of the stranded pigtail conductor I1, the overlapping portions of the two elements I1 and 24 being telescoped within a metal assembly sleeve 28. This sleeve is crimped adjacent the upper and lower ends thereof, as indicated at.28a and 28h, and the lower end of the fusible element 24 is brought out through the strands of the conductor I1 and bent over the lower edge of the sleeve 28 so that the portion 24h thereof overlaps the outer surface of the sleeve. After the three elements 28, 24, and I1 have been assembled to occupy the relative positions illustrated in Fig, 2 of the drawings, the crimps 28a and 28h may be formed around the upper and lower portions of the sleeve 28 for the purpose of providing a rigid mechanical connection between the three named parts. Thereafter, the lrwer end portion of the sleeve 28, the adjacent po'tion of the pigtail conductor I1 and the overlapping end portion 24h of the fusible element 24 nay be soldered to provide a rigid connection therebetween.

The fusible element 25 may more properly be designated a combination inductance element and heating element in that it serves the purpose of preventing surge currents of large magnitude from traversing the fusible elements 23 and 24 and also acts to heat the fusible element 23 to a fusing temperature when the link is subjected to an overload current for a sustained time inter val. In order to perform these two functions the element 25 is constructed in the form of a helical coil, and the upper end portion thereof is electrically and mechanically connected to the inner side wall surface of the tube 22 by means of a body of high melting point solder 25c. The turns of the element 25 are spaced apart axially of the tube 22, and the spaced apart relationship between the turns is maintained by embedding the same in a body of dielectric refractory material 21. This body is preferably formed of a refractory cement and serves several functions which are pointed out with particularity below. It may, for example, be formed of Portland cement or any ceramic material which is chemically inert, has high specific heat, and is possessed of good electrical insulating properties. In order to increase the inductance of the element 25, thereby to enhance the surge current blocking function thereof, particles 32a of magnetic material, such, for example, as iron powder or magnetite, may be dispersed throughout the body 21, but in no case should the density of the magnetic particles be such as to provide conductive paths capable of short-circuiting the turns of the element 25 or connecting any one of these turns with the tube 22. Among other functions, the body 21 serves rigidly to position or support the turns of the element 25 within the tube 22, and to this end entirely lls the upper portion of the tube. It also serves to support a tubular conductive connecting element 26 centrally of the tube 22, this element being utilized in the connection of the fusible element 24 with the lower end portion of the combination heating and inductance element 25. More specifically, the upper tubular portion of the connecting element 26 is projected within the turns of the element 25 in spaced apart relationship therefrom, and is embedded within the body 21. At its lower end this connecting element is provided with an outwardly extending flange 26a which serves to seat the lower turn of the element 25 in a manner such that the tubular portion of the element 26 is substantially concentrically disposed within the turns of the element 25. This lower turn of the element 25 is electrically and mechanically connected to the anged portion 26a of the element 26 by means of a high melting point, solder 25h, or the like. The upper end of the connecting element 26 is electrically and mechanically connected to the upper serpentine end 24a of the fusible element 24 through the fusible element 23, the latter element being in the form of a, body of alloy solder having a melting point of approximately 365 F.' It is to be noted that the fusible element 23 as thus formed within the tubular portion of the connecting element 25 is disposed well within the turns of the element 25 so that heat generated by current conduction through the latter element may be transferred to the fusible element 23 through that portion of the refractory body 21 which separates the fusible `element 23 from the adjacent turns of the element 25.

For the purpose of rapidly withdrawing the end 24a of the fusible element 24 from the connecting element 26 in the event the named fusible element is heated to a melting temperature, and the additional purpose of widening a break in the fusible element 24 cccasioned by the heating of this element to a fusing temperature, a coil spring 3i of the compression type is held under compression between the upper end of the sleeve 28 and the lower flange end 26a of the connecting element 26. More specifically, the coil spring 3l surrounds that portion of the fusible element 24 which extends between the upper end of the sleeve 28 and the lower end of the connecting element 25, a washer 30 formed of insulating material and provided with an enlarged central bore being interposed between the upper end of the spring and the flange 26a in order to prevent the fusible portion of the element 24 from being short-circuited through the turns thereof.

In order to maintain the turns of the coil spring 3| substantially concentrically disposed relative to the fusible element 24 and to maintain the sleeve 28 out of contact with the inturned end 22h of the tubular casing 22, the coil spring 3i and that portion of the sleeve 28 which extends within the tube 22 are surrounded by a tube 2S formed of Bakelite or other suitable insulating material. This tube fits snugly within the tubular member 22, and the lower end thereof abuts the inturned end 22h of the sleeve tube, thereby xedly to support the tube 29 within the tube 22. At its upper end, the insulating tube 29 is provided with an annular recess 29a for seating the flange 26a therewithin, thereby to partitionihe upper half of the tubular casing 22 from the lower half thereof, and to position the tubular connecting element 26 centrally of the tubular casing member 22.

In the assembly of the above described parts of the fuse link I0, the soldered connection between the upper end 24a of the fusible element 24 and the tubular portion of the connecting element 26 is rst made through the fusible body 23, following which the heating element 25 is telescoped over the tubular portion of the connecting element 26 and soldered to the flange portion 26a thereof. Following these operations, the insulating washer 30 and the coil spring 3| are telescoped over the lower end of the fusible element 24, and the lower end of this element is inserted through the sleeve covered strands of the pigtail conductor I 1, with the end 24h thereof being brought out and bent over the edge of the sleeve 28. In this regard it will be understood that the fusible element 24 is pulled through the sleeve covered end of the pigtail conductor 4I1 to masas compress the coil spring Il between the upper end of the sleeve 2l and the insulating washer 20, all before the end 2lb of the element 24 is locked over the lower edge of the sleeve 2l. The next operations are those of crimping the sleeve 28 at the portions 22a and 2Gb thereof, and of providing asoldered connection between the lower ends of the sleeve 28, the end portion 2lb of the element 24 and that portion of the pigtail conductor I'l which is adjacent the lower end of the sleeve 20. As a separate assembly operation, the insulating tube 28 is inserted into the tubular casing member 22 through the open upper end thereof until it abuts the inturned lower edge portion 22b thereof. The fusible element assembly may now be positioned within the tube 22 by threading the pigtail conductor l1 through the insulating tube 29 and the lower open end of the tube Z2. Movement of these elements through the tube 22 is obviously arrested when the flange portion 26e of the connecting element 26 is moved into engagement with the bottom of the annular recess 28a located at the upper edge of the insulating tube 29. After the fusible elements have thus been assembled within the tube 22, the upper end portion 25a may be soldered to the upper inner peripheral portion of the tube 22 as indicated at 25C, care being exercised to maintain the turns o! the element 25 disposed substantially concentrically within the tube 22. The next operation is that of illling the upper cavity formed within the tube 22 with the refractory cement or the 'mixture of refractory cement and magnetic particles. rihis may be accomplished by simply pouring the cement within the tube through the upper end thereof until all portions oi the element 25 are embedded therein. During this pouring operation the connector il@ is also embedded within the cement, and the cement is prevented from entering the lower chamber within the tube 22 due to the sealing engagement oi the conhector flange 26a with the ledge iormed by the bottom surface of the recess 29e. lifter the body of refractory cement has been allowed to set, or has been batted to enhance the setting operation, an appropriate die assembly operation may be .utilized to attach the terminal head l@ 'to the upper end oi the tube 2li. Following the last mena tioned operation the fuse linlr i@ is fully as* sembled and ready for use.

lin considering the mode oi operation of the fuse link ld, lt may be assumed that this link is to be serially included in the primary circuit of the transformer lil for the purpose of protecting this transformer aaglnst damage occasioned by current overloads. it may be assumed further that the transformer i9 is provided with a low voltage secondary load "which under normal conditions approximates the full load capacity ci the transformer lil, and that this secondary load includes motors and other devices which, during the start ing periods thereof, are capable of producing transient currents of relatively short duration in both the primary and secondary windings of the transformer ill. in this regard, it will be understood that due to the heat radiating capabilities of the transformer parts, the transformer I9 may be capable of withstanding an overload current of reasonable magnitude such, for example, as 20o to 300 per cent for a relatively long time interval; Whereas it can withstand current of the order of 500 per cent of normal for only a short time interi/al. lt will also be understood that the transformer is capable of being damaged by a sustained increase in the "voltage appearing between the conductors 20 of the sup ply circuit.

The fuse link i0 operates to protect the trans'- former against damages occasioned by overload currents caused by any one of the mentioned factors. At the same time, the fuse link permits the transformer to be operated under sustained overload current ccndltions'for a period less than that required for damage to the transformer, and will not blow when subjected to the normal and non-injurious high currents which are produced incident to motor starting, or the like. In this regard it will be understood that since the three serially related elements 25, 23, and 24 of the link I0 are traversed by the current flowing through the primary winding I9a of the transformer I9, they are all heated by current conduction and that the temperature of each element varies with changes in the magnitude of this current. The fusible element 23 is also heated by the heat conducted thereto from the element 25 through the walls of the connecting element 26. Heat energy is also transferred from the turns of the element '25 to the fusible element 23 through that r portion of the refractory body 2l which is disposed between the tubular portion of the connecting element 26 and the surrounding turns of the element 25. Under constant load current conditions and with a constant voltage between the conductors 20 of the current supply circuit, the amount of heat energy transferred to the fusible element 23 per unit time element interval remains substantially constant. naccordingly, so long as the load current through the secondary Winding 09h of the transformer does not substantially exoecd the iull load capacity oi this transformer, the fusible element it is operated at a temperature well below that required to produce fusion thereof. When, however, the secondary load current of the transformer lil gradually rises and is sustained for a period approaching that ai; which the transformer will be damaged, the temperature oi the fusible element 5.23 is raised accordingly. lThus, as the load current increases, the current traversing the three elements 2d, and l5 is correspondingly increased so that more heat is produced in the fusible element 523 by current conduction. Concurrentiy the amount of heat conducted to this fusible element from the ble elements and is increased. Also, concurrently, the amount of heat transerred iront the turns of the element to the fusible ele1 ment through the refractory body 2l is in= creased. .after a predetermined time interval, reduit-ed lor the accumulation oi heat within. the fusible element 23, this element is heated to fusing temperature and melts. "When this oc-l curs, the fusible element the sleeve and the upper end of the niatall conductor lll, quickly expelled irom the lower end of the tube 2li under the influence oi the coil spring 3l, thereby rapidly to orealr the circuit for energizing the primary winding i3d of 'the transformer lli.

To consider the action. of the fuse linls i@ ther, it may be pointed out that the refractory body 2l! prevents the fusible element from be= lng heated to its melting temperature when surge currents are produced in the primary circuit of the transformer l@ as a result of motor starting or the lilse. Such. surge currents are of short duration. being of the order of only a few seconds. The resulting momentary increase in heat generation within the fusible element is wholly insmdcient 'to raise the temperature of this elernent toits melting point. Moreover, that lnortlon of the refractory body 21 which is disposed between the tubular portion of the connecting element 25 and the turns of the heating element 25 dissipates a large portion of the heat resulting from the current surge through the element 25 away from the element 24. It also delays the transmission of the increased increment of heat produced by the element 25 to the adjacent walls of the' connecting element 26` for an interval which will normally exceedthe duration of the current transient. Accordingly, the increased increment of heat energy arriving at the surface of the connecting element 26 from the element 25 as a result of the momentary high current, effects an insufficient increase in the temperature of the fusible element 23 to cause this element to melt. In other words, the total heat accumulated in the fusible element 23 as a result of the transient high current is insufficient to heat this element to its fusing temperature. Thus it will be understood that the refractory body 21, or more exactly the thermal impedance of this body, protects the fusible element 23 against outages occasioned by transient currents of the character which frequently occur in the load pattern of any transformer secondary load. This is accomplished moreover, without increasing the thermal capacity of the fusible element 23 to a point such that it will provide no protection for sustained overload currents.

The thermal impedance of the refractory body 21 also prevents the fusible element 23 fromimmediately rupturing when thetransformer I9 is subjected to a high current, such, for example, as that which is produced when the secondary winding I9b of the transformerls is short-circuited. In the absence of an additional protective element, therefore, the transformer I9 could easily be damaged by an overload current of this character during the period required to transfer sufcient heat from the heating element 25 to the fusible element 23 to cause the latter element to melt. The second fusible element 24 functions to guard the transformer I 9 against damage when subjected to an overload current of this type. Thus, immediately the element 24 is subjected to a transient current of the particular character just referred to, a portion thereof lying between the upper end of the sleeve 23 and the lower end of the fusible element 23 is heated to a fusing temperature, permitting this element to rupture. As a result, the lower portion of the element 24 is expelled from the lower end of the tube 22 under the influence of the coil spring 3l. In this regard, it will be understood that the heat generated within that portion of the fusible element 24 which is surrounded by the sleeve 28, is rapidly conducted away from the enclosed portion of the element 24 by the strands of the pigtail conductor I1, whereby this portion of the element 24 is prevented from fusing when subjected to the described surge current of large magnitude.

As previously indicated, the elements 24 and 25 are so constructed that the inherent time-current characteristics vthereof are substantially the same. It may be pointed out, however, that the body of refractory material 21 has substantial heat capacity; whereas the -air which surrounds the fusible element 24 has relatively little heat capacity. Accordingly, the heat generated by the fusible element 25 as a result of the heavy surge current flow therethrough is absorbed by the body 21 at a relatively high rate, while the heat generated by the surge current flow through the fusible element 24 is only slowly absorbed by the air which surrounds this fusible element. As a result, the temperature of the fusible element 24 rises much more rapidly .than the temperature of the heating element 25, whereby fusing of the element 24 is insured before the element 251s heated to its melting temperature. Thus the fusible element 24 not only acts asa heavy'current-short interval protective device for the transformer I9, but in addition, functions to protect the heating element 25 against damage when the fuse link I0 is subjected to heavy surge current overloads.

As previously indica-ted, the annular surge gap 32, as provided between the sleeve 28 and the lower end of the metal tube 22, serves to protect the fusible elements of the link I0 and more particularly the element 24'thereof, against rupture when voltage transients of short duration but high value and steep wave front appear between the conductors 20 of the high voltage current supply circuit. In this regard it will be understood that lightning or other disturbances may cause voltage transients to appear between the line conductors 20 which are of insufficient duration to cause damage to the transformer I S and yet may be of sufficient magnitude to cause the fast responding fusible element 24 of the link I0 to rupture. When a voltage transient of `this character appears between the line conductors 20, the voltage across the turns Aof the element 25 instantly rises to a value sumcient to cause ionization of ythe shun't connected surge gap 32. If the wave front of the voltage transient is suiiiciently steep, this breakdown of the surge gap 32 will occur before the fusible element 24 can be damaged. Once the gap 32 is ionized, the electrical resistance thereof immediately drops to.an exceedingly low value such that the predominant portion of the surge current is by-passed around the fusible elements 23, 24, and 25 of the link I0. dies out, the voltage between the elements 22 and 28 decreases to a value insufficient to sustain ionization of the gap 32, with the result that surge current flow across this gap is arrested. From the above explanation it will be understood that the impedance of the coiled heating element 25 delays the build-up of current flow through the fusible `elements 23, 24, and 25, for an interval sufficient to insure breakdown of the surge gap 32 before the -current traversing the fusible element 24 can reach a magnitude sufiicient to cause this fusible element to rupture. Moreover, the particles 32 of magnetic material dispersed throughout the refractory body 21, in enhancing the electrical inductanlce of th-e heating element 25, serve to increase the delay. in current build-up through this element and the two serially related elements 23 and 24 when a voltage transient of the character described appears between the line conductors 20.

More generally considered, the time-current characteristics of one and two ampere fuses both characterized by the features of the invention described above, are respectively indicated at A and B in the graph shown in Fig. 3 of the drawings. In this graph, current through the fusible elements of the link, as measured in amperes, is plotted as a function of melting time in seconds, with the current being plotted on a logarithmic scale along the vertical axis of the graph and the melting time being plotted on a logarithmic scale along the horizontal axis of the graph. From an examination of the characteristic curve A, for example, for a fuse link having a l ampere rating, it will be seen that this curve is a composite of the curves AI and A2, individually and respective- As the transient c l l ly representing the time-currentI characteristics of the fusible elements 25 and The dash line projections of the `two characteristic curves Ai and A2 intersect .at the point D, such that :the solid line portions thereof, which extend in opposite directions away from the point D, represent the only portions of the Itwo curves over which the two fusible elements 23 :and 2d are operative to perform their protective functions. Thus, the curve AI, considered'in its entirety, indicates that the fusible element 24 is capable of conducting a sustained current of roughly o amperes for an in-1 denite period. Accordingly, that portion of the curve A2 which extends from the point D and intersects the l ampere ordinant of the graph, indicates the action of the fusible element 23 to provide sustained current overload protection. This portion of the curve A2 indicates that the link l@ will pass one ampere for substantially an lindennite period of time but that when the current through the link in increased to approximately 11/2 amperes, for example, the fusible element 23 will pass the overload current for not more than 100 seconds after which it will rupture. From a consideration of the dash line portion of the curve A2, it will be noted that the fusible element 23 provides substantially no protection during current overload time intervals oi less than l second regardless of the magnitude of the overload current. In the short time regions of the composite curve A, therefore, the fusible element must taire over the function of providing the required overload current protection. This it does, as will be seen from the curve AI. Thus, at currents in excess of amperes, the fusing tiine for the fusible element 24 :decreases rapidly with increasing current flow therethrough until a point E is reached :at which the surge gap t2 is broken down. In this regard it will be seen thta when a voltage transient is impressed across the fuse link i@ having a sume ciently steep wave front to increase the current through the link to a value in excess of a thousand amperes in a time interval of less than approximately .0001 second, the gap 32 is ionized .to 'bypass rthe surge current around the fusible elements of the link before any portion or the link can be heated to a melting temperature. lf the surge gap 32 is omitted from the link structure, the curve AI, representing the current-time characteristic of the fusible element 2t, may be projected on a straight line away from the point D along the path indicated until it inetrsects the vertical ordi nate representing the shorest time interval. The characteristic curve B for a fuse link having the higher current rating of 2 amparos conforms in configuration and pattern to the curve A analyzed above, but is, of course, lower than the curve A because of the greater current rating of the fuse linlr which it characterizes.

Referring now more particularly to i oi the drawings, there is illustrated a modified ar= rangement of the fuse linir wherein those parts which correspond to parts shown in 2 are identified by the same reference characters. in brief, the embodiment of the invention shown in Fic. 4 of the drawing differs from that shown in iig. 2 in that the extension spring Si is located outside of the tubular metal casing 227, and 'the flanged portion 26o of the tubular connecting element 26 is seated upon a small insulating washer 35 which rests upon the upper end of the insulating tube lMore specifically, the @XDDSOII Spring 3i surrounds lower end portion of the sleeve and, at its end, is tensioned and seated a tapered ille sulating washer 3G. This washer ls snugly. seated upon the inwardly and then outwardly curved end portion 22o oi the tube 22. Thus, the upper end of the spring 3l is restrained against transverse movement relative to the tube 22. At its lower end, this spring is anchored within curved fingers 28o which embrace the lower turn of the spring and are stuck out from the sides oi' the sleeve 2l at two or more points around the lower edge portion thereof. Aside from the difierences noted, the structure as shown in Fig. 4 oi the drawings is the same as that illustrated in Fig. 2 and described above, and the operating characteristics oi' the two structures are identical.

From the foregoing explanation it will be understood that the disclosed link structure is cx ceeciingly simple in arrangement, may be easily andi cheaply manufactured, and yet provides positive protection against damage to an associated transformer or the like when the link is subjected to all types of overload currents o1' the character normally encountered in operating practice. lt will also be noted that in several instances the individual elements of the structure are utilized to perform two or more functions. Thus the element 25 not only acts as a. heating element to impart the desired time-current characteristic to the fusible element 23, but additionally functions as an electrical impedance element to insure breakdown oi the surge gap 32 bollore the fusible elements 23 and 24 can be ruptured by a transient of large magnitude and steep wave front. Again, the body 2l of refractory material not only functions to render the fusible element 23 nonresponsive to surge currents of short duration, but additionally functions to protect the heating element 25 against burnouts or damage when the link is subjected to exceedingly heavy surge currents. This body, having the magnetic particles 22a dispersed therethrough, also functions to enhance the electrical inductance of the heating element 25 in order that this element may perform its surge current blocking function with increased effectiveness. Further, the element 2l not only functions as a surge current protective element, but additionally acts as a pullnout wire to break the electrical circuit through the fusible element 23 when the latter element n heated to a melting temperature.

While two embodiments of the invention have been disclosed, it will be understood that various modifications may be made therein without departing from the scope of the invention as set forth in the appended claims.

E claim:

l. A fuse linlr comprising a metal tubeya ter minal element positioned adjacent one end of said tube and spaced therefrom so that a surge cap is provided between the adjacent surfaces or said tube and said element, a combination heat ing and inductance element comprising a conductive coil disposed within said tube, and a fusible element electrically connected in series with said combination element across said gap and at least partially disposed within the turns oi? said coil in heat exchange relationship therewith.

2. A fuse link comprising a metal tube, a ter minal element positioned adjacent one end of said tube and spaced therefrom so that a surge nap is provided between the adjacent surfaces of said tube and said element, a combination heating and inductance element comprising a conductive coil disposed within said tube, a fusible element electrically connected in series with said combination element across said gap and at least partially disposed within the turns of said coil in heat exchange relationship therewith, and a body of refractory material at least partially supporting said fusible and combination elements within said tube and disposed therebetween.

3. In a fuse link, means defining a surge current path, a fusible element electrically connected in shunt with said path, and combination impedance and heating means arranged in heat exchange relationship with said element for causing surge currents to traverse said path and for heating said element to a fusing temperature when said link is subjected to a current overload of predetermined character.

4. A fuse link comprising a metal casing, means coacting with said casing to define a surge current path, a fusible element housed by said casing and electrically connected in shunt with said path, and combination impedance and heating means disposed within said casing in heat exchange relationship with said element for causing surge currents to traverse said path and for heating said element to a fusing temperature when said link is subjected to a current overload of predetermined character,

5. In a fuse link, a metal tube provided with a flange at one end thereof, a coiled conductor surrounding said tube and provided with a terminal end which seats on and is connected to said flange, a second conductor extending Within said tube, a fusible metal body electrically and mechanically connecting said second conductor to the inner surface of said tube at the opposite end thereof, and a body of refractory material in which the turns of said conductor and said tube are at least partially imbedded.

6. In a fuse link, a metal tube, a coiled conductor surrounding said tube and provided with a terminal end which is connected to one end of said tube, a second conductor extending Within said tube, a fusible metal body electrically and mechanically connecting said second conductor to the opposite end of said tube, a tubular casing enclosing the named parts, a body of insulating material at least partially imbedding the turns of said first named conductor and said tube and filling a portion of the space within said casing, and magnetic material imbedded in said body of insulating material to venhance the inductance of said coiled conductor.

7. A fuse link comprising a tubular metal casing, means coacting with said casing to define a spark gap, a coiled heating element housed within said casing out of electrical contact therewith, and a fusible element at least partially disposed within said coiled heating element to receive heat therefrom and connected in series with said heating element across said gap, whereby the impedance of said heating element assists in producing sparkover at said gap when a predetermined voltage is applied to said link.

8. A fuse link comprising a tubular metal casing having an open end, means coacting with the open end of said casing to define a spark gap, a coiled heating element housed Within said casing out of electrical contact therewith, a fusible element at least partially disposed within said coiled heating element to receive heat therefrom and connected in series with said heating element across said gap, whereby the inductance of said heating element assists in producing sparkover at said gap when a predetermined voltage is applied to said link, and means interposed between said fusible element and said heating element for delaying the transfer of heat from said heating element to said fusible element.

9. A fuse link comprising a tubular metal casing having an open end, an insulating tube fitting within said casing, a tubular conductor having a flange supported out of -contact with said casing by said insulating tube, a second conductor disposed within said insulating tube and including a portion extending within said tubular conductor, a fusible body electrically and mechanically connecting said portion of said conductor to said tubular conductor, a coiled heating element at least partially surrounding said tubular conductor to deliver heat to said fusible body, said coiled heating element being connected in series with said fusible body and said second conductor, and a body of heat transfer material at least partially lmbedding said tubular conductor and the turns of said coiled heating element.

10. A fuse link comprising a tubular metal casing having an open end, an insulating tube fitting within said casing, a tubular conductor having a flange supported out of contact with said casing by said insulating tube, a second conductor disposed within said yinsulating tube and including a portion extending within said tubular conductor, a fusible Ibody electrically and mechanically connecting said portion of said conductor to said tubular conductor, a coiled heating element at least partially surrounding said tubular conductor to deliver heat to said fusible body, said coiled heating element being connected in series with said fusible body and said second conductor, a body of refractory heat transfer material at least partially imbedding said tubular conductor and the turns of said coiled heating element, and means electrically connected with said second conductor and coacting with said casing to define a spark gap which shunts said series connected fusible body and coiled heating element.

11. A fuse link comprising a first fusible element, a second fusible element having an end embedded in said first fusible element, a heating element surrounding said first fusible element to deliver heat thereto and electrically connected in series with said fusi-ble elements, a body of heat transfer material interposed between said heating element and said first fusible element to determine the rate of heat transfer between said heating element and said first fusible element, and means electrically coacting with at least one of said fusible elements to define a surge gap shunting said heating element.

12. A fuse link comprising a rst fusible element, a second fusible element having a serpentine end embedded in said first fusible element, a heating coil surrounding said first fusible element to deliver heat thereto and electrically connected in series with said fusible elements, a body of heat transfer material interposed between said heating element and said first fusible element to determine the'rate of heat transfer between said heating element and said first fusible element, and means electrically coacting with at least one of said fusible elements to define a surge gap shunting said heating coil.

13. A fuse link, comprising a tubular casing, an insulating tube extending through at least a part of said casing, a metal tube supported by said insulating tube within said casing, a coiled metete l5 heating element et least partially surrounding said metal tube and having e terminal electrically connected thereto, e. second conductor extending through said insulating tube into seid metal tube, a. fusible metal body electrically end mechanically connecting saioi secondi conductor to the inner surface o seid metal tube and adapted to receive heet from seid heating element, and a. body of heet treinstel material 'interposed between seid heating eiement and seid metal tube to control the rate of heet transfer from seid heating element to seid fusible metal body.

14. In a fuse link, e. pair of current conductive elements disposed in spaced epert relationship to define a surge gap, e. fusible element electricaliy connected to' at leest one of seid current conductive elements, end combination impedance and heating Ineens arranged in heat exchange relationship with seid fusible element for causing surge currents to traverse seid path enel for heating seid element to e, fusing tempera-,ture when saiai lint; is subjected to e. curieit overtoad of predetermined character.

REFERENCES CITED The following references are of record in the le of this patent:

UNTED STATES PATENTS Number Name Date 662,466 Sechs Nov. 27, 1900 1,730,716 Austin Oct. 8, 1929 1,909,665 Northrup Apr. 18, 1933 2,104,999 Earle Jan. 11, 1938 2,153,859 Horiloshi May 16, 1939 2,17%,476 Pittmen et el Sept. 26, 1939 2,281,029 Earle Apr. 28, 1942 2,305,399 Smith, Jr. Dec. 15, 1942 2,305,436 McMorrs Dec. 15, 1942 2,321,711 Taylor' June 15, 1943 PATENTS Number Country Date 579636 Germany May 12, 1933 

